Acquisition of projection images for tomosynthesis

ABSTRACT

Some aspects include acquisition of a first plurality of projection images of a volume using a megavoltage x-ray source, each of the first plurality of projection images associated with a respective one of a first plurality of locations of the megavoltage x-ray source, acquisition of a second plurality of projection images of the volume using a kilovoltage x-ray source, each of the second plurality of projection images associated with a respective one of a second plurality of locations of the kilovoltage x-ray source, and performance of digital tomosynthesis reconstruction to generate a three-dimensional image of the volume based on the first plurality of projection images and the second plurality of projection images. The first axis may be perpendicular to the second axis.

BACKGROUND

1. Field

The embodiments described herein relate generally to radiation-basedimaging systems. More particularly, the described embodiments relate toradiation-based imaging systems used in conjunction with radiationtherapy.

2. Description

A linear accelerator produces electrons or photons having particularenergies. In one common application, a linear accelerator generates aradiation beam and directs the beam toward a target area of a patient.The beam is intended to destroy cells within the target area by causingionizations within the cells or other radiation-induced cell damage.

Radiation treatment plans are intended to maximize radiation deliveredto a target while minimizing radiation delivered to healthy tissue. Todesign a radiation treatment plan, a designer must assume that relevantportions of a patient will be in particular positions relative to alinear accelerator during delivery of the treatment radiation. The goalsof maximizing target radiation and minimizing healthy tissue radiationmay not be achieved if the relevant portions are not positioned inaccordance with the treatment plan during delivery of the radiation.More specifically, errors in positioning the patient can cause thedelivery of low radiation doses to tumors and high radiation doses tosensitive healthy tissue. The potential for misdelivery increases withincreased positioning errors.

Conventional imaging systems may be used to verify patient positioningprior to and during the delivery of treatment radiation. Specifically,this verification is intended to confirm that relevant portions of apatient are positioned in accordance with a treatment plan. Some systemsmay generate, for example, a two-dimensional projection image of apatient portal by passing a radiation beam through the patient andreceiving the exiting beam at an imaging system (e.g., a flat panelimager). Other systems produce three-dimensional megavoltage cone beamcomputed tomography (MV CBCT) images and/or three-dimensionalkilovoltage cone beam computed tomography (kV CBCT) images of a patientvolume prior to and/or during radiation delivery thereto.Recently-developed systems include linear/arc tomosynthesis andstationary tomosynthesis, which provide three-dimensional images basedon fewer projection images than required by CBCT, but usually at poorerresolution.

Online Image-Guided Radiation Therapy (IGRT) refers to techniques inwhich a patient position is monitored in near real-time during radiationtreatment and/or between treatment intervals. These techniques thereforerequire imaging systems which are capable of generating images quickly.Moreover, the generated images should be suitably detailed to provideaccurate evaluation of the patient position. Conventional uses of theabove-described techniques fail to satisfy these requirements. Inparticular, conventional CBCT is too slow and complex, and imagesgenerated via tomosynthesis do not exhibit suitable depth resolution.

SUMMARY

To address at least the foregoing, some embodiments provide a system,method, apparatus, and means to acquire a first plurality of projectionimages of a volume using a megavoltage x-ray source, each of the firstplurality of projection images associated with a respective one of afirst plurality of locations of the megavoltage x-ray source, acquire asecond plurality of projection images of the volume using a kilovoltagex-ray source, each of the second plurality of projection imagesassociated with a respective one of a second plurality of locations ofthe kilovoltage x-ray source, and perform digital tomosynthesisreconstruction to generate a three-dimensional image of the volume basedon the first plurality of projection images and the second plurality ofprojection images.

The first plurality of locations may be evenly distributed about a firstaxis, the second plurality of locations are evenly distributed about asecond axis, and the first axis may be perpendicular to the second axis.In some aspects, the first axis and the second axis intersect at anisocenter of a linear accelerator. The first plurality of projectionimages and the second plurality of projection images may be acquiredcontemporaneously.

According to some aspects, a first plurality of projection images of avolume are acquired using a first plurality of imaging x-ray sources,each one of the first plurality of imaging x-ray sources disposed in afixed relation with respect to one another and to a first axis and toemit a respective imaging x-ray, and each of the first plurality ofprojection images associated with an imaging x-ray emitted by arespective one of the first plurality of imaging x-ray sources. A secondplurality of projection images of the volume are acquired using a secondplurality of imaging x-ray sources, each one of the second plurality ofimaging x-ray sources disposed in a fixed relation with respect to oneanother and to a second axis and to emit a respective imaging x-ray, andeach of the second plurality of projection images associated with animaging x-ray emitted by a respective one of the second plurality ofimaging x-ray sources. Digital tomosynthesis reconstruction is thenperformed to generate a three-dimensional image of the volume based onthe first plurality of projection images and the second plurality ofprojection images. Again, the first axis may be perpendicular to thesecond axis, and the first axis and the second axis may intersect at anisocenter of a linear accelerator.

Some aspects may be employed to acquire a first plurality of projectionimages of a volume using a first x-ray source, each of the firstplurality of projection images associated with a respective one of afirst plurality of locations of the first x-ray source within 22.5° of afirst axis, acquire a second plurality of projection images of thevolume using a second x-ray source, each of the second plurality ofprojection images associated with a respective one of a second pluralityof locations of the second x-ray source within 22.5° of a second axis,and perform digital tomosynthesis reconstruction to generate athree-dimensional image of the volume based on the first plurality ofprojection images, the second plurality of projection images and noother projection images.

In some aspects, the first axis is perpendicular to the second axis. Thefirst axis and the second axis may also or alternatively intersect at anisocenter of a linear accelerator. According to some aspects, the firstx-ray source comprises a megavoltage radiation source, and the secondx-ray source comprises a kilovoltage radiation source. Moreover, thefirst plurality of projection images and the second plurality ofprojection images may be acquired contemporaneously.

Aspects may include a first plurality of imaging x-ray sources, each oneof the first plurality of imaging x-ray sources disposed in a fixedrelation with respect to one another and to a first axis and to emit arespective imaging x-ray, and a first imaging system to acquire a firstplurality of projection images of a volume, each of the first pluralityof projection images associated with an imaging x-ray emitted by arespective one of the first plurality of imaging x-ray sources. Alsoincluded may be a second plurality of imaging x-ray sources, each one ofthe second plurality of imaging x-ray sources disposed in a fixedrelation with respect to one another and to a second axis and to emit arespective imaging x-ray, and a second imaging system to acquire asecond plurality of projection images of the volume, each of the secondplurality of projection images associated with an imaging x-ray emittedby a respective one of the second plurality of imaging x-ray sources. Aprocessor may perform digital tomosynthesis reconstruction to generate athree-dimensional image of the volume based on the first plurality ofprojection images and the second plurality of projection images.

The claims are not limited to the disclosed embodiments, however, asthose in the art can readily adapt the description herein to createother embodiments and applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The construction and usage of embodiments will become readily apparentfrom consideration of the following specification as illustrated in theaccompanying drawings, in which like reference numerals designate likeparts, and wherein:

FIG. 1 is a perspective view of a radiation treatment room according tosome embodiments;

FIG. 2 comprises a flow diagram illustrating a process according to someembodiments;

FIGS. 3A through 3C comprise front views of two imaging systems toillustrate a process according to some embodiments;

FIGS. 4A and 4B illustrate sampling in three-dimensional Fourier space;

FIG. 5 is a perspective view of a radiation treatment room according tosome embodiments;

FIG. 6 comprises a flow diagram illustrating a process according to someembodiments;

FIG. 7 comprises a front view of two imaging systems to illustrate aprocess according to some embodiments;

FIG. 8 is a perspective view of a radiation treatment room according tosome embodiments; and

FIG. 9 is a perspective view of a radiation treatment room according tosome embodiments.

DETAILED DESCRIPTION

The following description is provided to enable any person in the art tomake and use the described embodiments and sets forth the best modecontemplated by the inventors for carrying out the describedembodiments. Various modifications, however, will remain readilyapparent to those in the art.

FIG. 1 illustrates radiation treatment room 100 pursuant to someembodiments. Radiation treatment room 100 includes linear accelerator(linac) 110, table 120, beam object 130, imaging system 140 and operatorconsole 150. The elements of radiation treatment room 100 may be used todeliver a treatment beam of x-rays to a target volume of beam object130. In this regard, beam object 130 may comprise a patient positionedto receive the treatment beam according to a radiation treatment plan.The elements of treatment room 100 may be employed in other applicationsaccording to some embodiments.

Linac 110 may comprise a megavoltage radiotherapy delivery systemproviding kilovoltage imaging such as the ARTISTE™ system from SiemensMedical Systems, but embodiments are not limited thereto. Linac 110generates and emits a treatment x-ray beam from treatment head 111.Treatment head 111 includes a beam-emitting device for emitting a beamduring calibration, acquisition of projection images, and/or treatment.The radiation beam may comprise electron, photon or any other type ofradiation. According to some embodiments, the beam exhibits energies inthe megavoltage range (i.e. >1 MeV) and may therefore be referred to asa megavoltage beam.

Also included within treatment head 111 is a beam-shielding device, orcollimator, for shaping the beam and for shielding sensitive surfacesfrom the beam. The collimator may be rotated and various elements of thecollimator may be positioned according to a treatment plan. Thecollimator may thereby control a cross-sectional shape of the beam.

Treatment head 111 is coupled to a projection of gantry 112. Gantry 112is rotatable around gantry axis 113 before, during and after emission ofa megavoltage beam therefrom. As indicated by arrow 114, gantry 112 mayrotate clockwise or counter-clockwise according to some embodiments.Rotation of gantry 112 serves to rotate treatment head 111 around axis113.

During imaging or radiation treatment, treatment head 111 emits adivergent beam of megavoltage x-rays along beam axis 115. The beam isemitted towards an isocenter of linac 110. The isocenter is located atthe intersection of beam axis 115 and gantry axis 113. Due to divergenceof the beam and the shaping of the beam by the aforementionedbeam-shaping devices, the beam may deliver radiation to a volume of beamobject 130 rather than only through the isocenter.

Table 120 supports beam object 130 during radiation treatment. Table 120may be adjustable to assist in positioning a treatment area of beamobject 130 at the isocenter of linac 110. Table 120 may also be used tosupport devices used for such positioning, for calibration and/or forverification.

Imaging device 116 may comprise any system to acquire an image based onreceived x-rays, including but not limited to a flat panel imager.Imaging device 116 may be attached to gantry 112 in any manner,including via extendible and retractable housing 111. Rotation of gantry112 may cause treatment head 111 and imaging device 116 to rotate aroundthe isocenter such that the isocenter remains located between treatmenthead 111 and imaging device 116 during the rotation.

Imaging device 116 may acquire projection images based on radiationemitted from treatment head 111 before, during and/or after radiationtreatment. For example, imaging device 116 may be used to acquire imagesfor verification and recordation of a target volume position and of aninternal patient portal to which radiation is delivered. These imagesmay reflect the attenuative properties of objects located betweentreatment head 111 and imaging device 116. As will be described below,such projection images may be used to reconstruct a three-dimensionalimage of the objects located between treatment head 111 and imagingdevice 116.

Imaging system 140 includes kilovoltage x-ray source 141, imaging device142, support 143 and extension 144. Imaging system 140 may acquireprojection images of an object located between source 141 and imagingdevice 142. Imaging system 140 is coupled to gantry 112 via extension144. In some embodiments, support 143 is rotatably coupled to extensionto allow source 141 and device 142 to rotate about axis 113. Kilovoltagex-ray source 141 may comprise any suitable single or multi-source deviceto emit imaging radiation, including but not limited to a conventionalx-ray tube. In some embodiments, x-ray source 141 emits kilovoltageradiation having energies ranging from 50 to 150 keV.

Operator console 150 includes input device 151 for receivinginstructions from an operator such as an instruction to verify a patientposition and an instruction to deliver treatment radiation according toa treatment plan. Console 150 also includes output device 152, which maybe a monitor for presenting calculated projection images, acquiredprojection images, three-dimensional images, operational parameters oflinear accelerator 110 and/or interfaces for controlling elementsthereof. Input device 151 and output device 152 are coupled to processor153 and storage 154.

Processor 153 executes program code according to some embodiments. Theprogram code may be executable to control linear accelerator 110 tooperate as described herein. The program code may be stored in storage154, which may comprise one or more storage media of identical ordifferent types, including but not limited to a fixed disk, a floppydisk, a CD-ROM, a DVD-ROM, a Zip™ disk, a magnetic tape, and a signal.Storage 154 may store, for example, projection images, three-dimensionalimages, radiation treatment plans, software applications to operatelinear accelerator 110, and other data used to perform radiationtreatment.

Operator console 150 may be located apart from linear accelerator 110,such as in a different room, in order to protect its operator fromradiation. For example, linear accelerator 110 may be located in aheavily shielded room, such as a concrete vault, which shields theoperator from radiation generated by linear accelerator 110.

Each of the devices shown in FIG. 1 may include less or more elementsthan those shown. In addition, embodiments are not limited to thedevices shown in FIG. 1.

FIG. 2 is a flow diagram of a process according to some embodiments.Process 200 and the other processes described herein may be performedusing any suitable combination of hardware, software or manual means.Software embodying these processes may be stored by any medium,including a fixed disk, a floppy disk, a CD-ROM, a DVD-ROM, a Zip™ disk,a magnetic tape, or a signal. Examples of these processes will bedescribed below with respect to the elements of treatment room 100, butembodiments are not limited thereto.

Process 200 may be performed at any time, including during calibrationof during a radiation treatment fraction. In some embodiments, and priorto S201, an operator may manipulate input device 151 of operator console150 to initiate operation of linear accelerator 110 to execute aradiation treatment plan. In response, processor 153 may execute programcode of a system control application stored in storage 154. The operatormay then operate input device 151 to initiate a patient positioningprocedure requiring a three-dimensional image of a patient volume.

At S201, a first plurality of projection images of a volume is acquiredusing a megavoltage x-ray source. Each of the first plurality ofprojection images is associated with a respective one of a firstplurality of locations of the megavoltage x-ray source. Similarly, atS202, a second plurality of projection images of the volume is acquiredusing a kilovoltage x-ray source. Each of the second plurality ofprojection images is associated with a respective one of a secondplurality of locations of the kilovoltage x-ray source.

FIGS. 3A through 3C illustrate S201 and S202 according to someembodiments. According to FIGS. 3A through 3C, S201 and S202 occurcontemporaneously, but embodiments are not limited thereto. FIG. 3Aillustrates gantry 112 and imaging system 140 in respective “Home”positions (i.e., HomeA and HomeB, respectively) prior to S201.Embodiments are not limited to initial 0° home positions as shown.

FIG. 3B shows clockwise rotation of gantry 112 through angle θ_(A1) andcounterclockwise rotation of imaging system 140 through angle θ_(B1).During this rotation, some of the first plurality of projection imagesare acquired by emitting radiation from x-ray source 111 and receivingthe radiation (as attenuated by beam object 130) at imaging device 116.Also during the rotation, some of the second plurality of projectionimages are acquired by emitting radiation from x-ray source 141 andreceiving the attenuated radiation at imaging device 142. Angle θ_(A1)and angle θ_(B1) may be identical or different. According to someembodiments, angle θ_(A1) and angle θ_(B1) are less than 22.5°.

FIG. 3C shows counterclockwise rotation of gantry 112 through angleθ_(A1), past the HomeA position, and through angle θ_(A2), and clockwiserotation of imaging system 140 through angle θ_(B1), past the HomeBposition, and through angle θ_(A1). During this rotation, others of thefirst plurality of projection images and the second plurality ofprojection images are acquired by imaging device 116 and imaging device142, respectively. Again, angle θ_(A2) and angle θ_(B2) may be identicalor different, and may be less than 22.5°. Moreover, angle θ_(A1) andangle θ_(A2) need not be identical, and angle θ_(B1) and angle θ_(B2)need not be identical.

A total angle of rotation (e.g., θ_(A1)+angle θ_(A2)) of each imagingsystem may approach 45° to obtain a compete sampling of the volume, aswill be described below. In some embodiments of process 200, one ofgantry 112 and imaging system 140 may first rotate to acquire acorresponding set of projection images, followed by rotation of theother to acquire its corresponding set of projection images.

The scanning trajectories of imaging system 140 and the imaging systemof treatment head 111 and imaging device 116 are orthogonal, butembodiments are not limited thereto. More specifically, each trajectoryis associated with a respective central axis (i.e., HomeA and HomeB),and these axes are perpendicular to one another. The present inventorshave discovered that projection images acquired using two tomosynthesisscanning trajectories at an angle from one another may result inimproved three-dimensional images reconstructed therefrom.

Therefore, at S203, digital tomosynthesis is performed to generate athree-dimensional image of the volume based on the first set ofprojection images and the second set of projection images. According tosome embodiments, only projection images which are acquired within 22.5°of a scanning trajectory's central axis are used to generate thethree-dimensional image, because projection images acquired outside thisangular range may not provide any additional sampling as will bedescribed below. Various digital tomosynthesis reconstruction algorithmshave been developed, which include filtered back-projection anditerative reconstruction algorithms. For example, the projection imagesmay be filtered with a Ram-Lak filter before back-projection.

The different scanning trajectories provide more complete sampling ofthe volume in the Fourier domain than provided by conventionaltomosynthesis systems. Such sampling may reduce artifacts in thereconstructed three-dimensional image that would otherwise result fromconventional tomosynthesis systems.

FIG. 4A illustrates three-dimensional Fourier sampling resulting from asingle circular source tomosynthesis trajectory. Circular sourcetrajectories will be discussed below. As shown in FIG. 4A, the conicalregions are not sampled by the single trajectory. A three-dimensionalimage reconstructed based on this incomplete sampling will likelyexhibit artifacts and poor depth resolution.

In contrast, FIG. 4B shows three-dimensional Fourier sampling resultingfrom circular source tomosynthesis trajectories, which in this casehappen to be orthogonal and which do not deviate more than 22.5° fromtheir central axes. As shown, the three-dimensional Fourier space ismore completely sampled than in FIG. 4A, resulting in a more resolvedthree-dimensional reconstruction than would result from the FIG. 4Asampling.

Many implementations other than that shown in FIG. 1 may exhibit theforegoing advantages. FIG. 5 is a perspective view of treatment room 500including fixed tomosynthesis x-ray sources 519 and 541.

X-ray sources 519 are disposed in a plane perpendicular to axis 515 andare arranged in a circular configuration, but embodiments are notlimited thereto. In this regard, x-ray sources 519 may comprise anygeometrical arrangement and operate in any manner, including thosedescribed in commonly-assigned co-pending applications (Attorney docketnos. 2007P10274US01 and 2008P00307US), and in Fixed Gantry TomosynthesisSystem For Radiation Therapy Image Guidance Based On A Multiple SourceX-Ray Tube With Carbon Nanotube Cathodes, Maltz et al., Med. Phys. 36(5), May 2009, pp. 1624-1636.

X-ray sources 519 may comprise any sources known to emit kilovoltageradiation or other imaging radiation that are or become known. In someembodiments, x-ray sources 519 employ cathodes based on carbon nanotubeor thermionic emission technology. X-ray sources 519 are affixed togantry 512 such that each x-ray source 519 is disposed in a fixedrelationship to each other x-ray source 519. Moreover, in someembodiments, each x-ray source 519 is disposed in a fixed relationshipwith respect to treatment head 511.

Imaging device 516 may be used to acquire a projection image based onradiation emitted from each one of x-ray sources 519. Each of radiationsources 519 is oriented such that radiation emitted therefrom passesthrough an isocenter of linear accelerator 510 and on to imaging device516. The acquired projection images may reflect the attenuativeproperties of objects located between x-ray sources 519 and imagingdevice 516.

X-ray sources 519 exhibit a circular source trajectory. The trajectoryis associated with a central axis which is identical to beam axis 515.Embodiments are not limited thereto.

Imaging system 540 is similar to imaging system 140 of FIG. 1. However,in contrast to x-ray source 141, imaging system 540 includes x-raysources 541 arranged in a circular configuration such that each x-raysource 541 is disposed in a fixed relationship to each other x-raysource 541. X-ray sources 541 may also comprise any sources to emitkilovoltage radiation or other imaging radiation that are or becomeknown.

Support 543 may be rotatable upon extension 544, or may be fixed suchthat a central axis of x-ray sources 541 remains perpendicular to acentral axis of x-ray source 519 despite rotation of gantry 512. Asdescribed with respect to x-ray source 519, each of radiation sources541 is oriented such that radiation emitted therefrom passes through theisocenter of linear accelerator 510 and to imaging device 542 to createa projection image.

Process 600 may be performed by the FIG. 5 system according to someembodiments. Initially, at S601, a first plurality of projection imagesof a volume are acquired using a first plurality of imaging x-raysources. Each of the first plurality of imaging x-ray sources isdisposed in a fixed relation with respect to one another and withrespect to a first axis. Additionally, each of the first plurality ofprojection images is associated with an imaging x-ray emitted by arespective one of the first plurality of imaging x-ray sources.

Similarly, at S602, a second plurality of projection images of thevolume are acquired using a second plurality of imaging x-ray sources.Each of the second plurality of imaging x-ray sources is disposed in afixed relation with respect to one another and with respect to a secondaxis, and, each of the second plurality of projection images isassociated with an imaging x-ray emitted by a respective one of thesecond plurality of imaging x-ray sources.

FIG. 7 is a front perspective view of linear accelerator 510 forpurposes of describing an example of S601 and S602. Gantry 512 isrotated to a 45° position, but process 600 may be performed with gantry512 at any position. Central axes 515 and 545 of sources 519 and 541,respectively, are perpendicular to one another, but embodiments are notlimited thereto.

At S601, imaging device 516 acquires a projection image of object 130corresponding to each one of sources 519. For example, each of sources519 may emit radiation in succession and imaging device 516 may acquirea projection image based on each emission. In some embodiments, morethan one of sources 519 emits radiation simultaneously, imaging device516 receives the radiation, and known techniques are applied to generatea separate projection image corresponding to the radiation emitted byeach of the more than one sources.

Imaging device 542 acquires a projection image corresponding to each oneof sources 541 at S602. Again, each of sources 541 may emit radiationseparately and imaging device 542 may acquire a projection image basedon each separate emission. S601 and S602 may be performed substantiallysimultaneously (or contemporaneously if several steps are involved) inorder to reduce the time required for projection image acquisition.

Digital tomosynthesis is performed on the acquired projection images atS603 to generate a three-dimensional image of the volume. As describedwith respect to FIG. 4B, the three-dimensional image may be more usefulthan those acquired using previous techniques due to more completesampling of the Fourier space.

Some embodiments may employ any suitable combinations of theabove-described scanning trajectories. FIG. 8 Illustrates treatment room800 including sources 819 to provide projection images along a fixedcircular scanning trajectory as described with respect to sources 819.Treatment room 800 also includes imaging system 840 to provideprojection images along a linear arc scanning trajectory.

According to some embodiments, a three-dimensional image is generatedbased on a first set of projection images acquired using sources 819 andon a first set of projection images acquired using imaging system 840.According to some embodiments, the second set of projection images islimited to only those projection images acquired within 22.5° of acentral axis of the linear arc scanning trajectory of imaging system840.

FIG. 8 Illustrates treatment room 800 including sources 819 to provideprojection images along a fixed circular scanning trajectory asdescribed with respect to sources 819. Treatment room 800 also includesimaging system 840 similar to imaging system 140 to provide projectionimages along a linear arc scanning trajectory. The central axes of thesescanning trajectories may be perpendicular to one another (or in anotherrelationship) regardless of a position of gantry 812.

According to some embodiments, a three-dimensional image is generatedbased on a first set of projection images acquired using sources 819 andon a first set of projection images acquired using imaging system 840.According to some embodiments, the second set of projection images islimited to only those projection images acquired within 22.5° of acentral axis of the linear arc scanning trajectory of imaging system840.

FIG. 9 Illustrates treatment room 900 including treatment head 911 andimaging device 916 to provide projection images along a linear arcscanning trajectory as described with respect to treatment head 111 andimaging device 116. Imaging system 940, on the other hand, is to provideprojection images along a fixed circular scanning trajectory asdescribed with respect to imaging system 540. The central axes of thesescanning trajectories may be perpendicular to one another or may exhibitany other desired relationship regardless of a position of gantry 912.

As noted above, acquisition of a first set of projection images and asecond set of projection images according to any embodiment may occursuccessively, simultaneously, or with any degree of overlap. In additionto the above-described implementations, any employed imaging system maybe physically independent from other devices to any degree. That is,embodiments contemplate the use of one or more standalone C-arm imagingsystems, one or more standalone rings of fixed digital tomosynthesissources, standalone flat panel imagers, etc.

Those in the art will appreciate that various adaptations andmodifications of the above-described embodiments can be configuredwithout departing from the scope and spirit of the claims. Therefore, itis to be understood that the claims may be practiced other than asspecifically described herein.

1. A method comprising: acquiring a first plurality of projection imagesof a volume using a megavoltage x-ray source, each of the firstplurality of projection images acquired while the megavoltage x-raysource is located at a respective one of a first plurality of locationsof the megavoltage x-ray source within 22.5° of a first axis; acquiringa second plurality of projection images of the volume using akilovoltage x-ray source, each of the second plurality of projectionimages acquired while the kilovoltage x-ray source is located at arespective one of the second plurality of locations of the kilovoltagex-ray source within 22.5° of a second axis; and performing digitaltomosynthesis reconstruction to generate a three-dimensional image ofthe volume based on the first plurality of projection images and thesecond plurality of projection images.
 2. A method according to claim 1,wherein the first axis is perpendicular to the second axis.
 3. A methodaccording to claim 1, wherein the first plurality of projection imagesand the second plurality of projection images are acquiredcontemporaneously.
 4. A method according to claim 1, further comprising:emitting treatment radiation from the megavoltage x-ray source to thevolume.
 5. A method according to claim 2, wherein the first axis and thesecond axis intersect at an isocenter of a linear accelerator.